Power supply systems must not only provide operative power to loads during normal operating conditions, but also during startup or initialization of power to the loads. The demand for smaller product size has caused an evolution from relatively simple linear power supplies to more complex switching power supplies. FIG. 1 illustrates a power supply 101. This power supply 101 has multiple outputs connected to multiple loads 103, 105, and 107. Each of these loads have both a startup power requirement and an operating power requirement. These power requirements are illustrated in FIG. 2.
In FIG. 2 various waveforms associated with the startup power and operating power requirement for each of the loads 103, 105, and 107 is shown. For simplicity, a single waveform V.sub.OUT 201 shows a voltage over time representative of each of several voltages driving the loads 103, 105, and 107. Note that the voltage slowly rises 203 from zero to a stable voltage 205. During the transition 203 the loads 103, 105, and 107 are demanding the aforementioned startup power. When the voltage stabilizes, as shown at reference number 205, the loads 103, 105, and 107 are demanding operative power.
I.sub.LOAD1 207, I.sub.LOAD2 213, and I.sub.LOAD"n" 219, represent load currents demanded by each of the loads 103, 105, and 107. I.sub.COMPOSITE 225 represents the combined, or composite load current demanded from the power supply 101. A startup portion of each of the current waveforms is represented by reference numbers 209, 215, 221, and 227. An operating portion of each of the current waveforms is represented by reference numbers 211, 217, 223, and 229. The power demanded from the power supply 101 is the product of the voltage, represented simply by V.sub.OUT 201, and the composite current I.sub.COMPOSITE 225. Computation will show a fairly high startup power requirement for this power supply because of the fairly high startup current requirements of the individual loads 103, 105, and 107. High startup current can be partially attributable to cross-conduction in CMOS (complementary metal oxide semiconductor) integrated circuits, and other loads that are operating in an indeterminate state until their control circuits are powered-up.
Physical size, and stress, thus field reliability, of a power supply is dependent on this startup power requirement. This is true for simple linear, and more complex switching type power supplies. Inherently, switching power supplies have more internal components that are effected by both the startup power requirement and the operating power requirement. Since switching power supplies inherently switch on and off current through certain reactive components, these relatively high startup power demands especially tax certain components. Particularly, capacitors have a ripple current rating associated with a capacitor's ability to thermally recover from a transient change in current through the capacitor. As transient power demand increases linearly, the capacitor increases volumetrically to safely provide this power demand. Also certain inductors, indigenous to switching power supplies, must also increase volumetrically so that the core elements are not saturated during these relatively high power demands during startup.
Also, active switching elements, typically a FET (field effect transistors) or other type of semiconductor switch, need to have sufficient bulk to handle the high startup power demand. In any case, the FET will need to operate at a higher temperature to account for this increased power demand during startup.
Further, prior art switching power supplies require that the above-mentioned components have to be over-sized because of a minimum startup voltage problem inherent in these designs. Essentially, a source voltage for the switching power supply must be sufficiently high for the regulator to startup and operate properly. This becomes problematic when the switching power supply has a fairly high startup current demand and a current limiting circuit that limits power dissipation in the switching power supply. When the source voltage is applied to the switching power supply, the output voltage of the switching power supply starts to build. Responsive to this building voltage, the connected loads start to draw current. If the current demand exceeds the current limit, then the output voltage will be caused to reduce to a level coincident with a predetermined maximum power dissipation. This means that if the current limit isn't set high enough then the switching power supply's output voltage will never reach the specified voltage--thus not start up correctly. To prevent this from happening, the current limit needs to be set high enough to support a low source voltage startup sequence under the maximum startup loading condition. Setting the current limit higher requires volumetrically larger capacitors, inductors and a larger FET. Significantly, setting the current limit higher to accommodate this high startup current demand, also increases the headroom necessary to accommodate short circuit protection. For instance, if operating current is 5 amps, and startup current is 10 amps, then short circuit test limit must be above 10 amps. The additional headroom associated with this short circuit test limit will cause the aforementioned components to grow even larger and dissipate more power--thus heat.
As mentioned earlier, the switching power supply must also dissipate this extra power associated with the short circuit test limit, under a low source voltage condition during startup. Typically, the startup current associated with a switching power supply may be 200-300% of the maximum operating current. As mentioned earlier, this causes the inductors and capacitors to grow substantially in size. Also, the active switching element needs to be significantly oversized to survive this substantial startup current.
What is needed is an improved power supply system that is relatively compact, reliable, easily manufacturable, and can efficiently manage these relatively high startup power demands.